The present invention is directed to a spindle plug assembly for communicating pressurized air through an axle spindle to a rotary union assembly. Automatic tire inflation systems, also known as central tire inflation systems, are commonly used on tractor trailers and utilize compressed air as a source of pressurized air to fill a leaking tire while the trailer is in motion. The use of such inflation system is not limited to tractor trailers, however, but can be employed on any non-driven axle. References is made herein to tractor trailers as that is the most common use of such systems.
In an automatic tire inflation system, air is directed from the control box to the leaking tire through one of the trailer's axles, which either carries an air line from the control box, or is sealed and functions as an air conduit. Pressurized air carried by the axles communicates with each pair of trailer tires mounted thereon through a plug assembly located at the end of the pressurized axle. The pressurized air is communicated through a rotary union assembly, which in turn directs the air to valve stems on the rotating tires. A preferred rotary union assembly is disclosed in U.S. Pat. No. 6,105,645 entitled “Rotary Union Assembly For Use In Air Pressure Inflation Systems For Tractor Trailer Tires”, issued to Anthony L. Ingram. Applicant incorporates herein by reference the specification therein. Between each rotary union assembly and its associated tires, pressure responsive valves are employed so that upon the occurrence of a leak in one of the tires, the resulting pressure loss will cause one of the valves to open and allow air flow from the rotary union assembly to pass therethrough to the leaking tire.
A plug filter assembly located at the end of the pressurized axle communicates pressurized air carried by the axles to each pair of trailer tires mounted thereon through the rotary union assembly by which air flow is directed from a stationary air line to the valve stems on the rotating tires. The pressurized air in the axles communicates with the plug filter assembly, through an air passage which extends through the assembly into the pressurized axle, on the one end, and into the wheel lubrication compartments adjacent to the ends of the axles on the other end. The pressurized air is then communicated with each pair of trailer tires mounted thereon through the rotary union assembly.
While these central tire inflation systems are well known and in widespread use, communication between the pressurized air in the hollow axle spindles and the plug filter assembly has suffered several shortcomings. For example, press plug air passage tube systems frequently employ a cone shaped press plug with an outside o-ring seal that must be forcibly installed in the spindle end of the axle. This can cause damage to the o-ring seal and the interior wall of the spindle during installation. When the o-ring is damaged, the seal is compromised which can allow air into the wheel lubrication compartment. Any air leakage into the wheel lubrication compartment causes air pressure build up within the compartment which can damage the oil seals therein and create an oil leak. If the wheel bearings lose their lubrication, they will seize up and can cause a fire.
Press plug removal for maintenance or cleaning of the axle presents another problem. The interior of a trailer axle accumulates moisture from the air passing therethrough which causes rusting, creating debris in the form of rust particles within the axle that will clog the filter. The press plug must be forcibly removed to clean out the axle and prevent continued plugging of the filter carried thereby. The filter element must also be periodically cleaned or replaced. The removal of the press plug can not only destroy the press plug, it can damage the interior of the spindle to such an extent that it will not properly seal about a new press plug thus preventing a central tire inflation system being used on that axle in the future.
In response to these needs, a novel press-expansion plug filter assembly was developed that overcame many of the existing problems. That assembly is the subject of U.S. Pat. No. 6,394,556. It employed threaded fasteners that, upon tightening, effected expansion of a plastic head portion of the assembly against the interior wall of the axle spindle to removeably secure the assembly in place within the spindle. The assembly also included a threaded receptacle in the upstream end of a tubular body portion to which a filter element was threadably secured. To remove the device to clean the axle and/or clean or replace the filter, it was only necessary to loosen the threaded fasteners. While that assembly represented a significant advancement in the art, a certain degree of care was required in re-securing the plug assembly. If the fasteners were unevenly torqued, the securement of the press plug assembly within the spindle and the seal created therewith could be adversely affected. If the fasteners were not adequately torqued, neither a proper seal or securement would be obtained. If the fasteners were over torqued the assembly could be damaged as the head portion required the use of an expansible material such as plastic.
It would be highly desirable if a spindle plug assembly could be developed which retained the advantages of this previously patented structure but which could be more easily and quickly installed; and could be installed without the need for special tools and without regard to relatively narrow torque specifications. It would also be desirable if such an assembly obviated the need to employ expandable materials, which are more susceptible to damage, and could be constructed of the same durable steel as the axle spindle. This would not only enhance the durability of the assembly, but avoid differences in coefficients of expansion and contraction between the various components of the plug assembly and the axle spindle which can adversely impact the seal formed between the plug assembly and axle spindle. The expansion plug of the present invention obtains these results.